Thousands of baby seals have died on a remote Australian island in the sub-Antarctic, with bird flu identified as the cause of death. Nearly 80 per cent of the southern elephant seal pup population on Heard Island, located about 4000 kilometres from the Australian mainland, was wiped out, according to new research published on Wednesday.
Research expedition reveals mass mortality
Wildlife ecologist Dr Julie McInnes was among a group of Australian researchers who travelled to Heard Island in October and December last year to investigate whether the highly pathogenic avian influenza had reached the island. At that time, the H5 high pathogenicity avian influenza (HPAI) was not known to have reached Australia or its sub-Antarctic territories. However, recent bird flu deaths on nearby French territories—the Kerguelen and Crozet islands, about 450 kilometres from Heard Island—prompted the research journey.
Upon arrival, the Australian Antarctic Division team encountered a mass-mortality event. The researchers recorded an estimated 13,359 seal pup deaths on Heard Island, and viral genome analysis confirmed that the animals were infected with Influenza A H5N1. The researchers noted that the number of deaths could have further increased, as the mass-mortality event was still ongoing when they completed their surveys. Seal deaths were increasing at a rate of 5.6 per cent each day.
High mortality rates among seal groups
The average mortality rate among the seal groups, or harems, while researchers were on the island was about 76 per cent, with the highest mortality rate among the harems reaching 97 per cent. The findings, which have not yet been peer reviewed, were published in the science communication archive bioRxiv on Wednesday. Researchers say more seal pup deaths than the 13,359 reported likely occurred on the island, as the mass-mortality event continued.
Heard Island, a glacial and volcanic island, is a critical breeding sanctuary for birds and seals in the Southern Ocean. It is free from introduced predators and unoccupied by humans. The Australian Antarctic Division manages the World Heritage protected site, and human visitation has been rare since the 1850s, when sealing devastated the local elephant seal population.
Impact on other species
Researchers found that seals were not the only animals affected by the bird flu. They detected HPAI in six of the nine species they tested. Researchers also suspect that HPAI is behind a spike in deaths among the local king and gentoo penguin populations. Several hundred dead adult king penguins were found across the island in January.
Phylogenetic analysis, a type of biological mapping used to trace genes and diseases, traced the virus back to the Crozet Islands. Researchers believe it was introduced from the French territory to the southeast of Heard Island in August 2025. Their data shows a continued easterly spread of HPAI around the sub-Antarctic.
